Funds are being requested to purchase a Rigaku AFC5R four-circle automatic diffractometer with rotating anode x-ray generator and low temperature device. The machine will replace a fourteen year old Nicolet Pl-bar diffractometer and home-made cooling device, which are becoming more and more undependable and difficult to maintain. The Rigaku, as the Nicolet before it, will be the mainstay of data collection for x-ray structure analyses of: (1) synthetic DNA oligonucleotides, being examined in order to learn the ways in which base sequence influences local helix geometry in a manner that could be recognized by drugs or by proteins such as repressors; (2) complexes of DNA oligomers with groove-binding antitumor drugs, being studied in order to learn how to design drug analogues capable of targeting at any desired base sequence; (3) complexes of DNA oligomers with small control proteins such as repressors and restriction enzymes; (4) small designed proteins synthesized explicitly to test our understanding of the process of protein folding; (5) electron transfer enzymes; (6) multiple- protein photosynthetic reaction centers; and (7) the enzyme lysozyme at various temperatures from 37 C to 75 C, in order to understand heat stability and possible conformational changes preceding denaturation.